With reference to FIG. 1, pitot tubes are well known devices used in the calculation of the relative velocity of fluid flow relative to a body 9 (e.g., the velocity of the body through the fluid). A basic pitot tube generally consists of a base portion 11 and a barrel 13. The barrel typically is in the form of an extended hollow tube that faces directly into the direction of incoming fluid flow 15. The base portion supports and positions the barrel with respect to the body such that the fluid flow at the barrel is not affected by the body (e.g., the barrel is not within the boundary layer of the fluid flowing by the body).
Within the pitot tube, the flow of fluid is stopped, allowing for a pressure measurement of the stagnation pressure (also known as the total pressure), which is the sum of the static pressure and the dynamic pressure of the fluid flowing by the pitot tube. The pressure might be measured by a pressure sensor 17 in the pitot tube and transmitted by a wire 19, or the pressure might be measured in another location using a tube to put the pitot tube in fluid communication with that measurement location.
The static pressure can be separately measured at a static pressure port 21 that faces in a direction normal to the direction of fluid flow. The static pressure port can be in an entirely separate location, and use a separate pressure sensor 23 and wire 25 to report the static pressure.
With reference to FIG. 2, a pitot tube may alternatively be configured as a pitostatic tube. In this case, there is again a base 31 supporting a barrel 33. However, the pitot tube also includes a static port 35.
As was previously noted, the total pressure is the sum of the static and dynamic pressures. Under Bernoulli's equation, the dynamic pressure is a function of fluid velocity (and vice versa). Therefore, using an airspeed calculation processing system 27 (FIG. 1), the velocity may be calculated using the difference between the static and total pressures. The static and total pressures can be separately measured, as depicted in FIG. 1. Alternatively, as depicted in FIG. 2, the difference between the two can be directly sensed using a pressure sensor 37 set between two chambers, one being at the static pressure and the other being at the total pressure.
While there are many industrial uses for a simple pitot tube, these uses do not generally face the complexities presented to aircraft pitot tubes. On an aircraft, a pitot tube must be configured to deal with extreme variations in weather conditions, including rain, humidity and temperature. Moreover, because the calculation of airspeed is critical to safely operating an aircraft, the functionality of an aircraft pitot tube can literally be critical to the safety of the aircraft, its passengers and its cargo.
Referring again to FIG. 1, the barrel 13 of a typical pitot tube for a large aircraft might be ten inches long and a half inch wide. A distal end 41 of the barrel forms an opening that faces into the fluid flow, while a proximal end 43 is affixed to the base portion 11, which forms a pressure chamber 45 in fluid communication with the opening. The total pressure of the airflow may be measured within the pressure chamber. The pressure chamber may be provided with a comparatively small drain 47 at its gravitational bottom to allow for moisture to be drained when there is a pressure gradient between the tube pressure and the outside pressure. The drain is sized small enough allow the drainage without significantly impacting the total pressure measurement within the pressure chamber.
In some modern variations, pitot tubes may include complex arrangements of baffles and/or a variety of passages to provide for the total pressure, and in some cases static pressure, to be measured and/or compared. Also, active devices such as heaters may be used to prevent water from freezing prior to draining. As a result, complex pitot devices are sometimes used to help mitigate the possibility of blockage.
Because unmanned aerial vehicles (“UAVs”) are sometimes substantially smaller than manned aircraft, and because they may fly at substantially lower airspeeds, pitot tubes for UAVs may face environmental problems not typically faced by their larger versions, such as from small airborne contaminants that could clog up a very small drain. Thus, the small pitot tubes on small unmanned aircraft are susceptible to failure from being exposed to water, or even very high humidity levels. Such small UAVs are used in both military field situations and civilian applications. Each of these applications can have important functions that cannot wait for inclement weather to improve.
In one approach to developing a small UAV pitot tube that is less susceptible to water-based malfunction, a membrane vent has been deeply embedded within a pitot tube between the barrel and the base portion. The membrane is a microporous expanded polytetrafluoroethylene membrane, and allows the free passage of gases and vapors, while water, dust, dirt, and such are repelled. Properly constructed, such a vent can even allow the pitot tube to be submerged without exposing the sensor to water. Moreover, it can filter out contaminants (e.g., dust and dirt) from the air while remaining operational. Unfortunately, while the addition of the membrane does block out water and contaminants, it does not eliminate water-based malfunctions of the pitot tube.
Accordingly, there has existed a need for a small UAV pitot device capable of functioning in a wide variety of weather conditions. Preferred embodiments of the present invention satisfy these and/or other needs, and provide further related advantages.